Equipment and technique
An advanced ultrasound machine with a high frequency transducer (7.5–12 MHz) is the basic equipment required. High frequency transducers allow superior near field resolution and form the basis of characterization of benign and malignant thyroid nodules. Colour flow applications are now standard, and a high sensitivity colour flow and power Doppler system is ideal. When using colour flow and power Doppler the machine should be calibrated to allow depiction of slow flowing vessels in the head and neck.
In evaluating the thyroid gland, scanning in the transverse and longitudinal planes is the most commonly used method. Adequate extension of the neck is required to ensure complete assessment of the inferior aspect of the thyroid gland, though this may be difficult in the elderly. Adjusting the depth and gain settings is essential to ensure the whole of both lobes and the superficial isthmus are fully assessed. To evaluate large goitres a lower frequency (5 MHz) transducer may be required to assess extension into the retroclavicular/retrosternal region.
Ultrasound examination of the thyroid must always include a detailed examination of the neck for any cervical lymphadenopathy. Metastatic cervical lymph nodes are frequently seen in thyroid cancers and may affect the surgical management and prognosis of patients.
Ultrasound features of thyroid nodules
The vast majority of thyroid nodules are benign, and the role of a radiologist in assessment of the thyroid gland is to differentiate a malignant thyroid nodule from the more commonly seen benign ones. It is therefore important to evaluate the sonographic features of thyroid nodules as these aid in their characterization.
Echogenicity
The incidence of malignancy is 4% when a solid thyroid nodule is hyperechoic. If the lesion is hypoechoic (Fig. 1), the incidence of malignancy rises to 26% [29]. However, hypoechogenicity alone is inaccurate in predicting malignancy, and if used as a sole predictive sign, it has a relatively poor specificity (49%) and positive predictive value (40%) [30].
 | Figure 1 Longitudinal grey scale sonogram shows a solid, hypoechoic thyroid nodule (arrows) with ill-defined margins anteriorly. Histology: papillary carcinoma. |
Margins
A malignant thyroid nodule tends to have ill-defined margins on ultrasound (Fig. 1). A peripheral halo of decreased echogenicity is seen around hypoechoic and isoechoic nodules and is caused by either the capsule of the nodule or compressed thyroid tissue and vessels [31]. The absence of a halo has a specificity of 77% and sensitivity of 67% in predicting malignancy [32].
Calcification
Fine punctate calcification (Fig. 2) due to calcified psammoma bodies within the nodule is seen in papillary carcinoma in 25%–40% of cases [16]. If used as the sole predictive sign of malignancy, microcalcification is the most reliable one with an accuracy of 76%, specificity of 93% and a positive predictive value of 70% [30]. Coarse, dysmorphic or curvilinear calcifications commonly indicate benignity (Fig. 3).
 | Figure 2 Longitudinal grey scale sonogram shows characteristic punctate calcification (arrowheads) within an ill-defined solid hypoechoic thyroid nodule (arrows) which is highly suggestive of papillary carcinoma. |
 | Figure 3 Longitudinal grey scale sonogram shows coarse calcifications (arrows) with dense shadowing within a thyroid nodule suggestive of benign calcification. |
Comet tail sign
The presence of a comet tail (Fig. 4) sign in a thyroid nodule indicates the presence of colloid within a benign colloid nodule [33] and is a strong predictor of benignity.
 | Figure 4 Transverse grey scale sonogram shows the presence of comet-tail artifacts (arrowheads) within a predominantly cystic thyroid nodule (arrows). Features are of a benign colloid nodule. Curved arrow identifies the internal jugular vein and asterisk marks (more ...) |
Solid/cystic
It is generally believed that thyroid nodules with large cystic components are usually benign nodules that have undergone cystic degeneration or haemorrhage (Fig. 5). However, papillary carcinoma occasionally demonstrates a cystic component and may mimic a benign nodule, though the presence of punctate calcification within the solid component helps in its identification (Fig. 6).
 | Figure 5 Longitudinal grey scale sonogram shows a well-defined heterogeneous thyroid nodule (arrows) with a large cystic component (arrowheads) and septation (open arrows). Features are compatible with a benign hyperplastic nodule. |
 | Figure 6 Transverse grey scale sonogram shows a cystic component (open arrows) within a papillary carcinoma (arrows) of the thyroid. The presence of punctate calcification (arrowheads) identifies its malignant nature. |
Multinodularity
It is a myth that multinodularity implies benignity, as approximately 10%–20% of papillary carcinomas may be multicentric [31, 34]. In those with true solitary nodules confirmed at surgery the risk of cancer is the same as in those with multinodular goitres [35]. Therefore against a background of multinodular changes, extra caution should be taken not to miss a suspicious nodule.
Colour flow patterns
In general there are three patterns of vascular distribution within a thyroid nodule [36]:
- –Type I: complete absence of flow signal within the nodule
- –Type II: exclusive perinodular flow signals
- –Type III: intranodular flow with multiple vascular poles chaotically arranged, with or without significant perinodular vessels.
Type III pattern is generally associated with malignancy. Types I and II are more commonly seen in benign hyperplastic nodules [36, 37]. Unfortunately if used as the sole predictor of malignancy, colour flow characteristics are not accurate [32], and have to be used in combination with other features seen on grey scale ultrasound.
It is well recognized that the predictive ability of ultrasound for malignancy is effective only when multiple signs are present in the same nodule. Although their predictive value increases in summation, it is at the cost of sensitivity [32].
Ultrasound of thyroid cancer
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